JP3708059B2

JP3708059B2 - Heat-shrinkable polylactic acid-based biaxially stretched film and method for producing the same

Info

Publication number: JP3708059B2
Application number: JP2002064821A
Authority: JP
Inventors: 義之鶴崎; 計介村井
Original assignee: Unitika Ltd
Current assignee: Unitika Ltd
Priority date: 2002-03-11
Filing date: 2002-03-11
Publication date: 2005-10-19
Anticipated expiration: 2022-03-11
Also published as: JP2003260736A

Description

【０００１】
【発明の属する技術分野】
本発明は、熱収縮性ポリ乳酸系二軸延伸フィルムに関し、特に、二軸方向への熱収縮性に優れたポリ乳酸系二軸延伸フィルムおよびその製造方法に関するものである。
【０００２】
【従来の技術】
従来より、機械的強力や耐熱性や寸法安定性に優れた材料としてポリエチレンテレフタレートやポリプロピレンが知られており、これらを用いた延伸フィルムが産業界で幅広く使用されている。
【０００３】
しかしながら、これらのプラスチックフィルムは、その使用後に廃棄処理される際に、焼却処理を行うと、焼却時の発熱量が高いためその処理中に焼却炉を傷める恐れがあり、埋め立てによる廃棄処理を行うと、これらのプラスチック類は、化学的、生物学的安定性のためにほとんど分解せずに残留する。そのため、近年の環境保全に対する社会的要求の高まりに伴い、微生物などにより分解可能な生分解性を有し、コンポストでの堆肥化処理が可能な生分解性を有する樹脂からなるフィルムが要求されている。生分解性樹脂の中でもポリ乳酸は、各種でんぷんや糖類などを発酵して得られる乳酸を重合した植物由来の原料で、最終的には再び炭酸ガスと水となって地球的規模で環境リサイクルされる理想的なポリマー原料として各種用途に利用され始めている。
【０００４】
しかしながら、ポリ乳酸は硬くて脆いという性質を有し、ポリ乳酸からなる無延伸フィルムは、強度や伸度が低く、耐衝撃性に劣るため、そのままでは成形体としての実用性が不足する。
【０００５】
そこでポリ乳酸の脆性を向上するために、一軸あるいは二軸延伸して配向させる方法が知られており、一般的には、機械的強力や衝撃強度の向上や改善を図るために、二軸延伸処理によりフィルム化される。このようなポリ乳酸系二軸延伸積層フィルムは、情報記録材料（磁気カード）、工業用パッケージ、農業用マルチフィルムなどに展開され、一部は実用化に至っているものもある。
【０００６】
しかしながら、これらのポリ乳酸系二軸延伸フィルムには、ポリプロピレン系二軸延伸フィルムに代表されるような機械的強力と熱収縮性とを併せ持つものではなく、いわゆる低熱収縮オーバーラッピング包装フィルムに関する実用化例はほとんどみられない。例えば、特開平７−２５６７５３号公報、特開平９−１８７８６３号公報には、ポリ乳酸系重合体からなる熱収縮性二軸延伸フィルムが開示されているが、いずれもプラスチック製ボトル結束用への展開として低温高熱収縮性を意図したものであり、ポリプロピレン系二軸延伸フィルムのように機械的強力と熱収縮性とを併せ持つものではなかった。
【０００７】
【発明が解決しようとする課題】
本発明は前記問題点を解決し、機械的強力に優れ、適度な二軸方向への熱収縮性を有するだけでなく熱収縮後の外観性にも優れ、寸法安定性に優れているため印刷性の良好な熱収縮性ポリ乳酸系二軸延伸フィルムおよびその製造方法を提供するものである。
【０００８】
【課題を解決するための手段】
本発明者らは、前記目的を達成するため鋭意研究を行った結果、特定のポリ乳酸系樹脂を特定の延伸条件下で二軸延伸処理することで、機械的強力を維持しながらも適度な二軸方向への熱収縮性を有し、かつ印刷やコーティング等の二次加工に耐えうる寸法安定性を有し、さらにシュリンク後の包装外観に優れたポリ乳酸系二軸延伸フィルムが得られることを見出し本発明に至ったものである。すなわち本発明は、ポリ乳酸系樹脂からなる二軸延伸フィルムであって、前記ポリ乳酸系樹脂はＬ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝１００／０〜９４／６（モル％）であり、前記フィルムの８０℃における縦方向及び横方向の熱収縮率が６％以下であり、前記フィルムの融点よりも１０℃低い温度における前記フィルムの縦方向または横方向の少なくとも一方向への熱収縮率が３０％以上であり、前記フィルムの融点よりも１０℃低い温度における前記フィルムの縦方向と横方向との熱収縮率比が１．０〜１．３の範囲であることを特徴とする熱収縮性ポリ乳酸系二軸延伸フィルムを要旨とするものである。
【０００９】
また、本発明は、Ｌ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝１００／０〜９４／６（モル％）であるポリ乳酸を主成分とするポリ乳酸系樹脂を溶融製膜し、フィルムの縦延伸倍率をＸ、横延伸倍率をＹとしたときに、前記Ｘが２．５倍以上であり、前記Ｙが２．０倍以上であり、ＸのＹに対する延伸倍率比（Ｘ／Ｙ）が０．６〜１．５の範囲となるように二軸延伸してフィルム化し、その後に１００〜１３０℃で３〜３０秒間の熱固定処理を行い、さらに２〜１０％の条件下で弛緩処理することを特徴とするポリ乳酸系二軸延伸フィルムの製造方法を要旨とするものである。
【００１０】
【発明の実施の形態】
本発明の熱収縮性ポリ乳酸系二軸延伸フィルムは、Ｌ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝１００／０〜９４／６（モル％）であるポリ乳酸系樹脂からなる必要がある。ポリ乳酸系樹脂に占めるＤ−乳酸の含有量が６モル％を越えると、ポリ乳酸系樹脂は明確な融点を示さなくなり、結晶性に乏しいものとなる。その結果、延伸時の厚み精度が著しく悪化し、なおかつ延伸後の熱セットによる配向結晶化が進行しなくなるため、フィルムの巻き取り時にフィルムに割れや裂けが発生するという問題が生じるだけでなく、二次加工の面でもフィルムテンションによるフィルム破断や、ブロッキングによるトラブルが発生する。また、Ｌ−乳酸を単独で使用してもよいが、Ｄ−乳酸が配合されているほうが結晶性が緩和され、製膜性の良いものが得られる。従って、本発明においては、Ｌ−乳酸とＤ−乳酸とが、（Ｌ−乳酸）／（Ｄ−乳酸）＝９９／１〜９５／５（モル％）の範囲で配合されていることが、より好ましい。なお、Ｌ−乳酸とＤ−乳酸とは、上記の割合で配合されていれば共重合体であってもブレンド体であっても良い。
【００１１】
ポリ乳酸系樹脂の数平均分子量は、５万〜３０万の範囲にあることが好ましく、より好ましくは８万〜１５万である。数平均分子量が５万未満であると、得られるフィルムは機械的強力に劣るものとなり、延伸工程や巻き取り工程での切断も頻繁に起こり、操業性の低下を招く。一方、数平均分子量が３０万を越えると、加熱溶融時の流動性が乏しくなって製膜性が低下する。
【００１２】
ポリ乳酸系樹脂を得るための重合法としては、縮合重合法及び開環重合法のいずれの方法を採用することも可能であり、分子量増大を目的として少量の鎖延長剤、例えばジイソシアネート化合物，ジエポキシ化合物，酸無水物等を使用してもよい。
【００１３】
また、製造工程あるいは二次加工工程でのハンドリング、フィルム走行性の面からアンチブロッキング剤を添加することもできる。アンチブロッキング剤とは、シリカ、二酸化チタン、タルク、アルミナ等の安定な金属酸化物、炭酸カルシウム、リン酸カルシウム、硫酸バリウム等の安定な金属塩、またはポリ乳酸に対して不活性な有機樹脂からなるいわゆる有機系ビーズが好適に使用できる。これらのアンチブロッキング剤はいずれか１種類を単独で用いても良く、また２種類以上を併用しても良い。
【００１４】
本発明の熱収縮性ポリ乳酸系二軸延伸フィルムは、オーバーラッピング包装等に好適な機械的強力と熱収縮性とを併せ持つように、上記のポリ乳酸系樹脂の結晶性、融点等の樹脂特性に合わせて延伸温度、延伸倍率、熱固定温度、リラックス率を適宜調整することにより、熱収縮率及び縦横の熱収縮率比を以下の範囲内にコントロールする必要がある。
【００１５】
オーバーラッピング包装等に使用されるフィルムには、印刷やコーティングなどの二次加工処理が行われる。印刷工程やコーティング工程におけるインキや接着剤溶剤の乾燥は、通常は８０℃付近で行われることが多いため、少なくとも８０℃の熱履歴による熱変形を抑える必要がある。従って、本発明においては、８０℃における縦方向及び横方向の熱収縮率を６％以下とする必要がある。８０℃における縦方向及び横方向の熱収縮率が６％を超えると、フィルムは寸法安定性に劣るものとなり印刷工程やコーティング工程で印刷ずれなどを生じ、包装後の外観が著しく損なわれることとなる。従って、８０℃における縦方向及び横方向の熱収縮率は、４％以下であることが好ましい。
【００１６】
また、フィルムの適度なシュリンク性という観点から、フィルムの融点より１０℃低い温度において、フィルムの縦方向または横方向の少なくとも一方向への熱収縮率が３０％以上であり、かつ、フィルムの縦方向と横方向との熱収縮率比が０．６〜１．５の範囲である必要がある。フィルム融点より１０℃低い温度においてフィルムの縦方向または横方向の少なくとも一方向への熱収縮率が３０％未満であると、十分なシュリンク性が得られず、包装後の外観はタイト感のないものとなり商品価値が低くくなる。また、フィルム融点より１０℃低い温度においてフィルムの縦方向と横方向との熱収縮率比が０．６未満あるいは１．５を超えるものであると、縦横間の熱収縮率差が大きいため、たるみや収縮しわが発生して、同様に商品価値の低いものとなる。なお、本発明においてフィルム融点より１０℃低い温度とは、シュリンク工程において適度なシュリンク性をもたらす加工温度を意味するものである。フィルムの融点は、使用されるポリ乳酸系樹脂のＬ−乳酸／Ｄ−乳酸の含有比に応じて異なるが、フィルム融点より１０℃低い温度での熱収縮率および熱収縮率比が上記の範囲にあることにより、二軸方向の熱収縮性を有し、かつシュリンク後の包装外観に優れたフィルムとすることができる。
【００１７】
また、本発明の熱収縮性ポリ乳酸系二軸延伸フィルムは、１００℃以上でかつフィルムの融点よりも１０℃低い温度以下の温度範囲において、フィルムの最大熱収縮応力が０．７ＭＰａ以上であり、フィルムの縦方向と横方向との最大熱収縮応力比が０．６〜２．０の範囲にあることが好ましい。前記の温度範囲において、フィルムの最大熱収縮応力が０．７ＭＰａ未満であると、十分な機械的強力が得られない傾向にあり、フィルムの縦方向と横方向との最大熱収縮応力比が０．６未満、あるいは２．０を超えると、同様に機械的強力に劣る傾向にある。従って、フィルムの最大熱収縮応力は１．０〜３．０ＭＰａの範囲であることがより好ましく、フィルムの縦横の最大熱収縮応力比は０．８〜１．６の範囲であることがより好ましい。
【００１８】
本発明の熱収縮性ポリ乳酸系二軸延伸フィルムには、必要に応じて、コロナ放電処理、プラズマ処理、火炎処理等の表面処理をしても良く、フィルムの表面処理は印刷性の向上の点から有効な手段である。中でもコロナ放電処理は簡便さの点から好ましい。コロナ放電処理は、フィルム製造工程中いわゆるオンラインに行っても、スリット時いわゆるオフライン時に行っても良いが、いずれの場合も二次加工における印刷性や接着剤の密着力の点から、処理面のぬれ張力が４０ｍＮ／ｍを超えるよう処理することが望ましい。
【００１９】
本発明の熱収縮性ポリ乳酸系二軸延伸フィルムには、必要に応じて顔料，酸化防止剤，可塑剤，紫外線吸収剤，滑剤，結晶核剤、帯電防止剤等を任意の割合で添加してもよい。
【００２０】
本発明の熱収縮性ポリ乳酸系二軸延伸フィルムの厚みは、特に限定されるものではなく、用途や要求性能や価格等によって適宜設定すればよいが、１０〜２００μｍ程度の厚さであるのが適当である。
【００２１】
以下に、本発明の熱収縮性ポリ乳酸系二軸延伸フィルムの製造方法について、一例を挙げて説明する。
本発明の熱収縮性ポリ乳酸系二軸延伸フィルムの製造方法は特に限定されるものではなく、例えば、Ｔダイ法、インフレーション法、カレンダー法等が挙げられるが、Ｔダイを用いて溶融混練して押出すＴダイ法が好ましい。
【００２２】
Ｔダイ法により製造する場合には、特定のポリ乳酸にさらに必要に応じて可塑剤、滑剤を適量配合したポリ乳酸系樹脂組成物を押出機ホッパーに供給し、押出機を例えば、シリンダー温度１８０〜２６０℃、Ｔダイ温度２００〜２５０℃に加熱し、溶融混練して押し出し、２０〜４０℃に制御された冷却ロールで冷却し、厚さ１００〜５００μｍの未延伸フィルムを得る。
【００２３】
未延伸フィルムの延伸方法としては、テンター方式による同軸二軸延伸法、ロールとテンターによる逐次二軸延伸法のいずれでもよい。また、延伸倍率や延伸軌跡を自由に選択できる点で、リニアモーター駆動方式を用いることも有効である。
【００２４】
未延伸フィルムに二軸延伸処理を行う際には、フィルムの縦延伸倍率をＸ、横延伸倍率をＹとしたときに、縦延伸倍率Ｘが２．５倍以上であり、横延伸倍率Ｙが２．０倍以上であり、ＸのＹに対する延伸倍率比（Ｘ／Ｙ）が０．６〜１．５の範囲となるようにして延伸処理することが必要である。縦延伸倍率Ｘが２．５倍未満である、あるいは横延伸倍率が２．０倍未満であると、十分な機械的強力が得られず、実用性に劣るものとなる。また、縦延伸倍率Ｘ及び横延伸倍率Ｙの上限は特に限定されるものではないが、フィルムに上記のような特定の熱収縮率及び熱収縮率比を発現させるためには、縦方向と横方向の延伸倍率比（Ｘ／Ｙ）が０．６〜１．５の範囲となるように延伸処理することが好ましい。縦方向と横方向の延伸倍率比が０．６未満あるいは１．５を超えると、フィルムの縦横の熱収縮率差が大きくなり過ぎて二軸方向にバランスした熱収縮特性が得られ難くなる。
【００２５】
上記の延伸処理が行われた後、８０℃付近での二次加工時の寸法安定性及びフィルム融点付近での適度なシュリンク性の発現のために、延伸倍率やフィルム走行速度に応じて１００〜１３０℃で３〜３０秒間の熱固定処理を行うことが必要である。
【００２６】
また、熱固定処理後に縦横の熱収縮率比を調整するため、２〜１０％の条件下で弛緩処理することが必要である。
このようにして得られた本発明の熱収縮性ポリ乳酸系二軸延伸フィルムは、菓子袋等の食品包装材料や、医薬品などの包装材料や、磁気テープ、磁気ディスク等の個包装あるいは集積包装に適した低熱収縮オーバーラッピング用途の包装材料として好適に使用でき、その他にもハム、ソーセージ等の食品包装、ＰＥＴボトル等のプラスチックボトルやガラスびんのラベルやキャップ等として好適に使用できる。またゴミとして廃棄された場合には、土壌中で微生物により分解され、自然環境、野生動物に対する環境負荷を軽減することができる。
【００２７】
【実施例】
次に、実施例に基づき本発明を具体的に説明するが、本発明はこれらの実施例のみに限定されるものではない。なお、実施例、比較例における各種物性値の測定は以下の方法により実施した。
（１）フィルムの融点（℃）：示差走査型熱量計（パーキンエルマー社製、ＤＳＣ−７型）を用いて、試料質量を１０ｍｇ、昇温速度を１０℃／分で測定し、得られた融解熱曲線において吸熱ピークが最大となる温度を融点とした。
（２）熱収縮率（％）：試料長（ＭＤ方向）が１５０ｍｍ、試料幅（ＴＤ方向）が１０ｍｍの試料片を作製し、この試験片を熱風乾燥機に８０℃で５分間熱処理した。そして、下記式より、試験片の８０℃における縦方向（ＭＤ方向）の熱収縮率と横方向（ＴＤ方向）の熱収縮率とをそれぞれ求めた。また、（フィルムの融点−１０）℃の温度で５分間熱処理したものについても同様にして縦方向（ＭＤ方向）と横方向（ＴＤ方向）の熱収縮率をそれぞれ求めた。
熱収縮率(%)＝［（熱処理前試料長−熱処理後試料長）／熱処理前試料長］×100
本発明においては、フィルムの縦方向（ＭＤ）と横方向（ＴＤ）との熱収縮率比（ＭＤ／ＴＤ）が０．６〜１．５の範囲にあるものを合格とした。
（３）最大熱収縮応力（ＭＰａ）：熱物理分析計（ＴＡインスツルメント社製、ＴＭＡ２９４０）を用い、幅４．５ｍｍに切り出したフィルムを室温からフィルム融点まで１０℃／分の昇温速度で昇温した際の最大応力を初期断面積（試料巾×厚み）で割ることにより求めた。
【００２８】
本発明においては、フィルムの縦方向（ＭＤ）と横方向（ＴＤ）との最大熱収縮応力比（ＭＤ／ＴＤ）が０．６〜２．０の範囲にあるものを合格とした。
（４）シュリンク性：磁気カセットテープケースを対象として、（フィルム融点−１０）℃でのシュリンク工程後のフィルム外観を目視により観察し、下記のように評価した。
【００２９】
○：しわの発生がなかったまたはタイト感があり見栄えがよかった
△：シール部のしわが目立ったまたは熱収縮不足によるだぶつきが見られた
×：しわが著しかったまたは外観のだぶつきが著しかった
本発明においては、評価が○であるものを合格とした。
（５）印刷性：ポリ乳酸系二軸延伸フィルムから幅５４０ｍｍ、長さ１００ｍの試験フィルムを切り出し、このフィルムの一方の面（表面処理面）に、２液ウレタンインキ（大日本インキ化学工業社製）を用いて２色のトンボの図柄をそれぞれ４０ｃｍピッチで印刷できるグラビアコータで印刷した。そして、トンボの間隔を測定して印刷ピッチのずれを求め、下記のように評価した。
【００３０】
印刷ピッチずれ
○：印刷ピッチずれが２ｍｍ以下であり、印刷ピッチのずれが殆ど認められなかった
△：印刷ピッチずれが２ｍｍを超え５ｍｍ以下であり、若干の印刷ピッチのずれが認められた
×：印刷ピッチずれが５ｍｍを超え、印刷ピッチのずれが著しかった
本発明においては、評価が○であるものを合格とした。
（６）総合評価：全ての項目が合格であり、二軸方向へのバランスした熱収縮性フィルムであったものを○、不合格の項目が有った、あるいは二軸方向へのバランスした熱収縮フィルムが得られなかったものを×で表した。
実施例１
ポリ乳酸として、数平均分子量が９５，０００、ＭＦＲが６．５ｇ／１０分（２１０℃）、Ｌ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝９８．８／１．２（モル％）であるポリ乳酸（カーギル・ダウ・ポリマー社製）を用いた。このポリ乳酸１００質量部に対し、アンチブロッキング剤として平均粒径が１．４μｍの不定形シリカ（富士シリシア化学社製、サイリシア３１０Ｐ）０．１質量部を配合した。
【００３１】
このポリ乳酸系樹脂組成物を、９０ｍｍφの口径を有する単軸押出機で２３０℃で溶融し、Ｔダイにてシート状に押し出し、同時に表面温度が２０℃のキャストロールで急冷固化して厚さ２３０μｍの未延伸フィルムを得た。樹脂の押し出し量は、後述の延伸倍率を考慮して、フィルム厚みが最終的に２５μｍとなるように調整した。
【００３２】
得られた未延伸シートを倍率可変型の同時二軸延伸機に供給して、ステンター内の予熱温度８５℃、延伸温度８０℃として、縦（ＭＤ）方向に３．０倍、横（ＴＤ）方向に３．０倍の延伸倍率となるように同時二軸延伸を行い、続いて１１７℃で１０秒の熱固定処理を行い、横方向のリラックス率を３％として、厚み２５μｍの二軸延伸フィルムを作製した。
【００３３】
得られたフィルムの物性等を表１に示す。
【００３４】
【表１】

実施例２
ポリ乳酸として、数平均分子量が９７，０００、ＭＦＲが６．０ｇ／１０分（２１０℃）、Ｌ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝９８．０／２．０（モル％）であるポリ乳酸（カーギル・ダウ・ポリマー社製）を用いた。また、ステンター内の予熱温度を８０℃、延伸温度を７８℃として、熱固定処理を１１５℃で１０秒間とした。そしてそれ以外は実施例１と同様にして二軸延伸フィルムを作製した。
【００３５】
得られたフィルムの物性等を表１に示す。
実施例３
ポリ乳酸として、数平均分子量が９４，０００、ＭＦＲが７．０ｇ／１０分（２１０℃）、Ｌ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝９５．５／４．５（モル％）であるポリ乳酸（カーギル・ダウ・ポリマー社製）を用いた。また、延伸温度を７７℃とし、熱固定処理を１１２℃で１０秒間とした。そしてそれ以外は実施例１と同様にして二軸延伸フィルムを作製した。
【００３６】
得られたフィルムの物性等を表１に示す。
実施例４
実施例３と同様にして厚み３００μｍの未延伸フィルムを作製した。この未延伸フィルムを、予熱ロール６５℃、延伸ロール７４℃とした逐次二軸延伸機を用いて縦方向に３．０倍延伸し、引き続いて８２℃の延伸温度で横方向に４．０倍延伸処理した。得られた延伸フィルムに横方向のリラックス率を７％として、１１５℃で１０秒間の熱固定処理を施し、二軸延伸フィルムを得た。
【００３７】
得られたフィルムの物性等を表１に示す。
実施例１〜４は、いずれもポリ乳酸を構成するＬ−乳酸とＤ−乳酸との割合が本発明の範囲内であり、このポリ乳酸の結晶性や融点等の樹脂特性に合わせて延伸温度、延伸倍率、熱固定温度、リラックス率を適宜調整したため、フィルムの８０℃における縦方向及び横方向の熱収縮率、フィルムの融点よりも１０℃低い温度におけるフィルムの少なくとも一方向への熱収縮率とフィルムの縦横の熱収縮率比を本発明の範囲内とすることができ、最大熱収縮応力及び最大熱収縮応力比が高く機械的特性に優れ、適度な熱収縮性を有し、しかも熱収縮後の外観性にも優れたシュリンク性の良いフィルムが得られ、さらに寸法安定性に優れており印刷性の良い二軸延伸フィルムが得られた。この二軸延伸フィルムは、いわゆる低熱収縮オーバーラッピング包装材料として好適に使用でき、その他にもハム、ソーセージ等の食品包装、ＰＥＴボトル等のプラスチックボトルやガラスびんのラベルやキャップ等として好適に使用できるものであった。
比較例１
ＭＤ方向への延伸温度を７０℃とし、延伸倍率を２．０倍とした。続くＴＤ方向への延伸温度を７８℃とし、延伸倍率を３．５倍とした。そしてそれ以外は実施例４と同様にして二軸延伸フィルムを得た。
【００３８】
得られたフィルムの物性等を表１に示す。
比較例２
ＭＤ方向への延伸温度を７０℃とし、延伸倍率を２．０倍とした。続くＴＤ方向への延伸温度を７６℃とし、延伸倍率を３．０倍とした。そしてそれ以外は実施例４と同様にして二軸延伸フィルムを得た。
【００３９】
得られたフィルムの物性等を表１に示す。
比較例３
ＭＤ方向への延伸温度を８０℃とし、延伸倍率を４．８倍とした。続くＴＤ方向への延伸温度を８５℃とし、延伸倍率を３．０倍とした。また、横方向のリラックス率を１０％とするとともに、熱固定処理を１２０℃で１０秒間とした。そしてそれ以外は実施例４と同様にして二軸延伸フィルムを得た。
【００４０】
得られたフィルムの物性等を表１に示す。
比較例４
ポリ乳酸として、数平均分子量が９０，０００、ＭＦＲが７．５ｇ／１０分（２１０℃）、Ｌ−乳酸とＤ−乳酸との割合が（Ｌ−乳酸）／（Ｄ−乳酸）＝８５．０／１５．０（モル％）であるポリ乳酸（カーギル・ダウ・ポリマー社製）を用いた。そしてそれ以外は実施例１と同様にして未延伸フィルムを得た。ステンター内の予熱温度を７０℃、延伸温度を６７℃とした以外は実施例１と同様にして得られた未延伸フィルムに同時二軸延伸処理を施した。次いで、リラックス率を５％として９０℃で１０秒間の熱固定処理を施そうとしたが、熱固定処理時に溶断を起こしてフィルム化できなかった。
比較例５
ポリ乳酸の代りに脂肪族ポリエステル（昭和高分子社製、ビオノーレ１９０３）を用いた。そしてそれ以外は実施例１と同様にして未延伸フィルムを得た。ステンター内の予熱温度を６０℃、延伸温度を６０℃とした以外は実施例１と同様にして得られた未延伸フィルムに同時二軸延伸を施した。次いで、横方向のリラックス率を５％として８０℃で１０秒間の熱固定処理を施そうとしたが、熱固定時に溶断を起こしてフィルム化できなかった。
【００４１】
比較例１は、縦延伸倍率が低く、かつ延伸倍率比が本発明の範囲よりも小さかったため、二軸方向にバランスした熱収縮性が得られず、機械的強力も低くなり、印刷性にも劣るものとなった。
【００４２】
比較例２は、縦延伸倍率が本発明の範囲よりも低かったため、シュリンク性に劣り、包装後の外観にタイト感がなく、印刷性にも劣るものとなった。
比較例３は、延伸倍率比が本発明の範囲を超えていたためフィルムの融点よりも１０℃低い温度における熱収縮比が本発明の範囲よりも大きくなり、シュリンク処理後のフィルムには、しわやだぶつきといった外観悪化が生じ、シュリンク性に劣るものとなった。
【００４３】
比較例４は、Ｌ−乳酸とＤ−乳酸の配合割合が本発明の範囲を外れるポリ乳酸を用いたため、また、比較例５は、本発明のポリ乳酸の代りに脂肪族ポリエステルを用いたため、いずれも延伸処理後のフィルムはステンター内で溶断を起こしてフィルム化できなかった。
【００４４】
【発明の効果】
以上のように本発明の熱収縮性ポリ乳酸系二軸延伸フィルムによれば、特定の条件下で二軸延伸処理を行うことで、機械的物性を損なうことなく、適度な二軸方向の熱収縮性を有し、かつ印刷性やシュリンク性に優れたフィルムが得られる。このような二軸延伸フィルムは、菓子袋などの食品包装材料や、医薬品などの包装材料、磁気テープ、磁気ディスク等の個別包装あるいは集積包装に適した低熱収縮オーバーラッピング包装材料や、プラスチックボトルやガラスびんに使用されるラベルやキャップとして好適に使用できる。またゴミとして廃棄された場合には、土壌中で微生物により分解され、自然環境、野生動物に対する環境負荷を軽減することができる。
【００４５】
また、本発明の熱収縮性ポリ乳酸系二軸延伸フィルムの製造方法によれば、本発明のポリ乳酸系二軸延伸フィルムを容易に実現できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heat-shrinkable polylactic acid-based biaxially stretched film, and more particularly to a polylactic acid-based biaxially stretched film excellent in heat-shrinkability in the biaxial direction and a method for producing the same.
[0002]
[Prior art]
Conventionally, polyethylene terephthalate and polypropylene are known as materials having excellent mechanical strength, heat resistance and dimensional stability, and stretched films using these are widely used in the industry.
[0003]
However, when these plastic films are incinerated after being used, if they are incinerated, the amount of heat generated during incineration is high, which may damage the incinerator during the processing. And these plastics remain almost undegradable due to chemical and biological stability. Therefore, with the recent increase in social demand for environmental conservation, there is a demand for a film made of a biodegradable resin that has biodegradability that can be decomposed by microorganisms and that can be composted in compost. Yes. Among biodegradable resins, polylactic acid is a plant-derived raw material obtained by polymerizing lactic acid obtained by fermenting various starches and sugars, and is finally recycled to the environment on a global scale using carbon dioxide and water again. It has begun to be used for various applications as an ideal polymer raw material.
[0004]
However, polylactic acid has the property of being hard and brittle, and an unstretched film made of polylactic acid has low strength and elongation and is inferior in impact resistance.
[0005]
Therefore, in order to improve the brittleness of polylactic acid, a method of aligning by uniaxial or biaxial stretching is known, and in general, biaxial stretching is performed in order to improve or improve mechanical strength and impact strength. Filmed by processing. Such polylactic acid-based biaxially stretched laminated films have been developed in information recording materials (magnetic cards), industrial packages, agricultural multi-films, etc., and some have been put into practical use.
[0006]
However, these polylactic acid-based biaxially stretched films do not have both mechanical strength and heat shrinkability as represented by polypropylene-based biaxially stretched films. There are few examples. For example, JP-A-7-267553 and JP-A-9-187863 disclose heat-shrinkable biaxially stretched films made of a polylactic acid-based polymer, both of which are for binding plastic bottles. The development is intended for low-temperature high heat-shrinkability, and it does not have both mechanical strength and heat-shrinkability unlike a polypropylene biaxially stretched film.
[0007]
[Problems to be solved by the invention]
The present invention solves the above-mentioned problems, has excellent mechanical strength, has not only moderate heat shrinkability in the biaxial direction, but also excellent appearance after heat shrinkage and excellent dimensional stability. A heat-shrinkable polylactic acid biaxially stretched film having good properties and a method for producing the same are provided.
[0008]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors have conducted a biaxial stretching treatment on a specific polylactic acid-based resin under specific stretching conditions, thereby maintaining an appropriate strength while maintaining mechanical strength. A polylactic acid-based biaxially stretched film that has heat shrinkability in the biaxial direction, has dimensional stability that can withstand secondary processing such as printing and coating, and has an excellent packaging appearance after shrinking. It has been found that this has led to the present invention. That is, the present invention is a biaxially stretched film made of a polylactic acid resin, and the polylactic acid resin has a ratio of L-lactic acid to D-lactic acid of (L-lactic acid) / (D-lactic acid) = 100 / 0 to 94/6 (mol%), and the film has a heat shrinkage rate of 80% or less in the longitudinal direction and transverse direction of 6% or less, and the film has a longitudinal direction of 10 ° C. lower than the melting point of the film. Alternatively, the thermal shrinkage rate in at least one direction of the transverse direction is 30% or more, and the thermal shrinkage ratio between the longitudinal direction and the transverse direction of the film at a temperature 10 ° C. lower than the melting point of the film is 1.0 to 1. A heat-shrinkable polylactic acid-based biaxially stretched film characterized by being in the range of .3 .
[0009]
In addition, the present invention provides a poly (polylactic acid) as a main component in which the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 100/0 to 94/6 (mol%). When a lactic acid-based resin is melt-formed, the longitudinal stretch ratio of the film is X, and the lateral stretch ratio is Y, the X is 2.5 times or more, the Y is 2.0 times or more, and X The film was stretched biaxially so that the draw ratio (X / Y) of Y to Y was in the range of 0.6 to 1.5, and then heat-fixed at 100 to 130 ° C. for 3 to 30 seconds. The gist of the present invention is a method for producing a polylactic acid-based biaxially stretched film , which is further subjected to relaxation treatment under conditions of 2 to 10% .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In the heat-shrinkable polylactic acid biaxially stretched film of the present invention, the ratio of L-lactic acid and D-lactic acid is (L-lactic acid) / (D-lactic acid) = 100/0 to 94/6 (mol%). It must be made of some polylactic acid resin. When the content of D-lactic acid in the polylactic acid resin exceeds 6 mol%, the polylactic acid resin does not show a clear melting point and is poor in crystallinity. As a result, the thickness accuracy at the time of stretching significantly deteriorates, and orientation crystallization by the heat setting after stretching does not proceed, so that not only the problem that the film breaks or tears during winding of the film occurs, On the secondary processing side, film breakage due to film tension and troubles due to blocking occur. In addition, L-lactic acid may be used alone, but when D-lactic acid is blended, the crystallinity is eased and a film having good film forming property is obtained. Therefore, in the present invention, L-lactic acid and D-lactic acid are blended in the range of (L-lactic acid) / (D-lactic acid) = 99/1 to 95/5 (mol%). More preferred. The L-lactic acid and the D-lactic acid may be a copolymer or a blend as long as they are blended in the above ratio.
[0011]
The number average molecular weight of the polylactic acid resin is preferably in the range of 50,000 to 300,000, more preferably 80,000 to 150,000. When the number average molecular weight is less than 50,000, the resulting film is inferior in mechanical strength, and cutting frequently occurs in the stretching process and the winding process, resulting in a decrease in operability. On the other hand, when the number average molecular weight exceeds 300,000, the fluidity at the time of heating and melting becomes poor, and the film forming property is lowered.
[0012]
As a polymerization method for obtaining a polylactic acid-based resin, any of a condensation polymerization method and a ring-opening polymerization method can be adopted, and a small amount of chain extender such as a diisocyanate compound or diepoxy is used for the purpose of increasing the molecular weight. Compounds, acid anhydrides and the like may be used.
[0013]
Moreover, an antiblocking agent can also be added from the surface of handling in a manufacturing process or a secondary processing process, and film runnability. The anti-blocking agent is a so-called metal oxide that is stable to silica, titanium dioxide, talc, alumina, or the like, a stable metal salt such as calcium carbonate, calcium phosphate, or barium sulfate, or an organic resin that is inactive against polylactic acid. Organic beads can be preferably used. Any one of these antiblocking agents may be used alone, or two or more thereof may be used in combination.
[0014]
The heat-shrinkable polylactic acid-based biaxially stretched film of the present invention has resin properties such as crystallinity and melting point of the above-mentioned polylactic acid-based resin so as to have both mechanical strength and heat shrinkability suitable for overlapping packaging and the like. It is necessary to control the heat shrinkage ratio and the longitudinal and transverse heat shrinkage ratios within the following ranges by appropriately adjusting the drawing temperature, the draw ratio, the heat setting temperature, and the relaxation rate.
[0015]
Films used for overlapping packaging and the like are subjected to secondary processing such as printing and coating. Since the ink and the adhesive solvent in the printing process and the coating process are usually dried at around 80 ° C., it is necessary to suppress thermal deformation due to a heat history of at least 80 ° C. Therefore, in the present invention, the heat shrinkage rate in the vertical and horizontal directions at 80 ° C. needs to be 6% or less. If the thermal shrinkage in the vertical and horizontal directions at 80 ° C exceeds 6%, the film will be inferior in dimensional stability, causing printing misalignment in the printing process and coating process, and the appearance after packaging will be significantly impaired. Become. Therefore, it is preferable that the heat shrinkage rate in the vertical direction and the horizontal direction at 80 ° C. is 4% or less.
[0016]
Further, from the viewpoint of an appropriate shrink property of the film, at a temperature lower by 10 ° C. than the melting point of the film, the heat shrinkage rate in at least one direction of the longitudinal direction or the transverse direction of the film is 30% or more, and the longitudinal direction of the film The heat shrinkage ratio between the direction and the lateral direction needs to be in the range of 0.6 to 1.5. When the thermal shrinkage rate in at least one direction in the machine direction or the transverse direction of the film is less than 30% at a temperature lower by 10 ° C. than the film melting point, sufficient shrinkage cannot be obtained, and the appearance after packaging has no tight feeling. It becomes a thing and the product value becomes low. Moreover, since the heat shrinkage ratio between the machine direction and the transverse direction of the film is less than 0.6 or more than 1.5 at a temperature lower by 10 ° C. than the film melting point, the difference in heat shrinkage ratio between the length and width is large. Sagging and shrinkage wrinkles occur, and the product value is similarly low. In the present invention, the temperature 10 ° C. lower than the film melting point means a processing temperature that brings about an appropriate shrink property in the shrink process. The melting point of the film varies depending on the content ratio of L-lactic acid / D-lactic acid in the polylactic acid resin used, but the heat shrinkage ratio and the heat shrinkage ratio at a temperature 10 ° C. lower than the film melting point are within the above ranges. By being in this, it can be set as the film which has the heat-shrinkability of a biaxial direction, and was excellent in the package external appearance after shrink.
[0017]
Further, the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention has a maximum heat shrinkage stress of 0.7 MPa or more in a temperature range of 100 ° C. or more and 10 ° C. lower than the melting point of the film. The maximum heat shrinkage stress ratio between the longitudinal direction and the lateral direction of the film is preferably in the range of 0.6 to 2.0. In the above temperature range, if the maximum heat shrinkage stress of the film is less than 0.7 MPa, sufficient mechanical strength tends not to be obtained, and the maximum heat shrinkage stress ratio between the longitudinal direction and the transverse direction of the film is 0. If it is less than .6 or exceeds 2.0, the mechanical strength tends to be inferior. Therefore, the maximum heat shrinkage stress of the film is more preferably in the range of 1.0 to 3.0 MPa, and the maximum heat shrinkage stress ratio in the vertical and horizontal directions of the film is more preferably in the range of 0.8 to 1.6. .
[0018]
If necessary, the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention may be subjected to surface treatment such as corona discharge treatment, plasma treatment, flame treatment, etc. The surface treatment of the film improves printability. This is an effective means. Among these, corona discharge treatment is preferable from the viewpoint of simplicity. The corona discharge treatment may be performed online during the film manufacturing process, or may be performed offline at the time of slitting, but in any case, from the viewpoint of printability and adhesive adhesion in the secondary processing, It is desirable to treat the wetting tension to exceed 40 mN / m.
[0019]
A pigment, an antioxidant, a plasticizer, a UV absorber, a lubricant, a crystal nucleating agent, an antistatic agent, etc. may be added to the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention at any ratio as necessary. May be.
[0020]
The thickness of the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention is not particularly limited, and may be set as appropriate depending on the application, required performance, price, etc., but the thickness is about 10 to 200 μm. Is appropriate.
[0021]
Below, an example is given and demonstrated about the manufacturing method of the heat-shrinkable polylactic acid-type biaxially stretched film of this invention.
The method for producing the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention is not particularly limited, and examples thereof include a T-die method, an inflation method, a calendar method, and the like. The T-die method of extruding is preferable.
[0022]
In the case of manufacturing by the T-die method, a polylactic acid resin composition in which a suitable amount of a plasticizer and a lubricant is further blended with a specific polylactic acid as necessary is supplied to an extruder hopper. It is heated to ˜260 ° C. and T-die temperature is 200 to 250 ° C., melt kneaded and extruded, and cooled with a cooling roll controlled to 20 to 40 ° C. to obtain an unstretched film having a thickness of 100 to 500 μm.
[0023]
The stretching method for the unstretched film may be either a coaxial biaxial stretching method using a tenter method or a sequential biaxial stretching method using a roll and a tenter. In addition, it is also effective to use a linear motor drive system in that the stretching ratio and the stretching trajectory can be freely selected.
[0024]
When biaxial stretching treatment is performed on an unstretched film, when the longitudinal stretching ratio of the film is X and the lateral stretching ratio is Y, the longitudinal stretching ratio X is 2.5 times or more, and the lateral stretching ratio Y is It is necessary to perform the stretching treatment so that the stretching ratio (X / Y) of X to Y is in the range of 0.6 to 1.5. If the longitudinal draw ratio X is less than 2.5 times, or the transverse draw ratio is less than 2.0 times, sufficient mechanical strength cannot be obtained and the practicality is inferior. The upper limits of the longitudinal draw ratio X and the transverse draw ratio Y are not particularly limited, but in order for the film to exhibit the above specific heat shrinkage ratio and heat shrinkage ratio, the machine direction and the transverse direction Stretching is preferably performed so that the stretching ratio (X / Y) in the direction is in the range of 0.6 to 1.5. When the draw ratio between the machine direction and the transverse direction is less than 0.6 or more than 1.5, the difference in the heat shrinkage ratio between the machine and the transverse direction of the film becomes too large, and it becomes difficult to obtain heat shrink characteristics balanced in the biaxial direction.
[0025]
After the above stretching treatment is performed, in order to develop dimensional stability at the time of secondary processing near 80 ° C. and moderate shrinkability near the film melting point, 100 to 100 depending on the stretching ratio and the film running speed. It is necessary to perform heat setting treatment at 130 ° C. for 3 to 30 seconds.
[0026]
Moreover, in order to adjust the heat shrinkage ratio between the length and width after the heat setting treatment, it is necessary to perform a relaxation treatment under the condition of 2 to 10%.
The heat-shrinkable polylactic acid-based biaxially stretched film of the present invention thus obtained is used for food packaging materials such as confectionery bags, packaging materials such as pharmaceuticals, individual packaging such as magnetic tapes and magnetic disks, or integrated packaging. It can be suitably used as a packaging material suitable for low heat shrink overlapping applications, and can also be suitably used as food packaging such as ham and sausage, plastic bottles such as PET bottles, and labels and caps for glass bottles. Moreover, when it is discarded as garbage, it is decomposed by microorganisms in the soil, and the environmental load on the natural environment and wild animals can be reduced.
[0027]
【Example】
Next, the present invention will be specifically described based on examples, but the present invention is not limited to only these examples. In addition, the measurement of the various physical-property values in an Example and a comparative example was implemented with the following method.
(1) Melting point (° C.) of film: obtained by measuring the sample mass at 10 mg and heating rate at 10 ° C./min using a differential scanning calorimeter (manufactured by PerkinElmer, DSC-7). The temperature at which the endothermic peak was maximum in the heat of fusion curve was taken as the melting point.
(2) Thermal contraction rate (%): A sample piece having a sample length (MD direction) of 150 mm and a sample width (TD direction) of 10 mm was prepared, and the test piece was heat-treated at 80 ° C. for 5 minutes. And the thermal contraction rate of the vertical direction (MD direction) and the horizontal direction (TD direction) in 80 degreeC of the test piece were calculated | required from the following formula, respectively. Further, the heat shrinkage rate in the machine direction (MD direction) and the transverse direction (TD direction) was also obtained in the same manner for the film that was heat-treated at a temperature of (melting point of film −10) ° C. for 5 minutes.
Thermal shrinkage (%) = [(sample length before heat treatment−sample length after heat treatment) / sample length before heat treatment] × 100
In the present invention, a film having a thermal shrinkage ratio (MD / TD) in the range of 0.6 to 1.5 in the longitudinal direction (MD) and the transverse direction (TD) was regarded as acceptable.
(3) Maximum heat shrinkage stress (MPa): Using a thermophysical analyzer (TA Instruments, TMA2940), a film cut into a width of 4.5 mm was heated at a rate of 10 ° C./min from room temperature to the film melting point. Was obtained by dividing the maximum stress when the temperature was raised by the initial cross-sectional area (sample width × thickness).
[0028]
In the present invention, a film having a maximum heat shrinkage stress ratio (MD / TD) in the range of 0.6 to 2.0 in the longitudinal direction (MD) and the transverse direction (TD) is regarded as acceptable.
(4) Shrink property: With respect to the magnetic cassette tape case, the film appearance after the shrinking process at (film melting point−10) ° C. was observed visually and evaluated as follows.
[0029]
○: Wrinkle was not generated or it was tight and it looked good Δ: Wrinkles in the seal part were conspicuous or loose due to insufficient heat shrinkage was observed ×: Wrinkles were severe or appearance was severe In the present invention, a product with an evaluation of “good” was regarded as acceptable.
(5) Printability: A test film having a width of 540 mm and a length of 100 m is cut out from a polylactic acid-based biaxially stretched film, and a two-component urethane ink (Dainippon Ink & Chemicals, Inc.) is applied to one surface (surface-treated surface) of this film. The two-color dragonfly pattern was printed with a gravure coater capable of printing at a pitch of 40 cm. Then, the interval between the registration marks was measured to determine the deviation of the printing pitch and evaluated as follows.
[0030]
Print pitch deviation ◯: Print pitch deviation is 2 mm or less and almost no print pitch deviation is observed Δ: Print pitch deviation is more than 2 mm and 5 mm or less, and slight print pitch deviation is recognized x: In the present invention in which the printing pitch deviation exceeded 5 mm and the printing pitch deviation was significant, the evaluation was “good”.
(6) Comprehensive evaluation: All items passed and the heat shrinkable film was balanced in the biaxial direction, ○, there was a failed item, or the heat was balanced in the biaxial direction Those in which a shrink film could not be obtained were represented by x.
Example 1
As polylactic acid, the number average molecular weight is 95,000, MFR is 6.5 g / 10 min (210 ° C.), and the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 98. Polylactic acid (manufactured by Cargill Dow Polymer Co.) that was 8 / 1.2 (mol%) was used. To 100 parts by mass of this polylactic acid, 0.1 part by mass of amorphous silica (manufactured by Fuji Silysia Chemical Ltd., Silicia 310P) having an average particle size of 1.4 μm was blended as an antiblocking agent.
[0031]
This polylactic acid-based resin composition is melted at 230 ° C. with a single-screw extruder having a diameter of 90 mmφ, extruded into a sheet shape with a T-die, and simultaneously quenched and solidified with a cast roll having a surface temperature of 20 ° C. An unstretched film of 230 μm was obtained. The extrusion amount of the resin was adjusted so that the film thickness was finally 25 μm in consideration of the stretching ratio described later.
[0032]
The obtained unstretched sheet was supplied to a variable biaxial simultaneous biaxial stretching machine, and the preheating temperature in the stenter was 85 ° C., the stretching temperature was 80 ° C., 3.0 times in the machine direction (MD), and the width (TD). Simultaneously biaxially stretching to a stretching ratio of 3.0 times in the direction, followed by heat setting treatment at 117 ° C. for 10 seconds, with a lateral relaxation rate of 3%, and biaxial stretching with a thickness of 25 μm A film was prepared.
[0033]
The physical properties and the like of the obtained film are shown in Table 1.
[0034]
[Table 1]

Example 2
As polylactic acid, the number average molecular weight is 97,000, MFR is 6.0 g / 10 min (210 ° C.), and the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 98. Polylactic acid (manufactured by Cargill Dow Polymer Co.) that was 0 / 2.0 (mol%) was used. The preheating temperature in the stenter was 80 ° C., the stretching temperature was 78 ° C., and the heat setting treatment was 115 ° C. for 10 seconds. Other than that, a biaxially stretched film was produced in the same manner as in Example 1.
[0035]
The physical properties and the like of the obtained film are shown in Table 1.
Example 3
As polylactic acid, the number average molecular weight is 94,000, MFR is 7.0 g / 10 min (210 ° C.), and the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 95. Polylactic acid (manufactured by Cargill Dow Polymer Co., Ltd.) that was 5 / 4.5 (mol%) was used. The stretching temperature was 77 ° C., and the heat setting treatment was 112 ° C. for 10 seconds. Other than that, a biaxially stretched film was produced in the same manner as in Example 1.
[0036]
The physical properties and the like of the obtained film are shown in Table 1.
Example 4
An unstretched film having a thickness of 300 μm was prepared in the same manner as in Example 3. This unstretched film was stretched 3.0 times in the longitudinal direction using a sequential biaxial stretching machine with a preheating roll of 65 ° C. and a stretching roll of 74 ° C., followed by 4.0 times in the transverse direction at a stretching temperature of 82 ° C. Drawing was performed. The obtained stretched film was subjected to a heat setting treatment at 115 ° C. for 10 seconds at a transverse relaxation rate of 7% to obtain a biaxially stretched film.
[0037]
The physical properties and the like of the obtained film are shown in Table 1.
In Examples 1 to 4, the ratio of L-lactic acid and D-lactic acid constituting polylactic acid is within the scope of the present invention, and the stretching temperature is adjusted in accordance with the resin properties such as crystallinity and melting point of this polylactic acid. Since the draw ratio, heat setting temperature, and relaxation rate were appropriately adjusted, the heat shrinkage rate in the longitudinal and transverse directions at 80 ° C of the film, and the heat shrinkage rate in at least one direction of the film at a temperature 10 ° C lower than the melting point of the film The ratio of the vertical and horizontal heat shrinkage ratios of the film and the film can be within the range of the present invention, the maximum heat shrinkage stress and the maximum heat shrinkage stress ratio are high, the mechanical properties are excellent, the heat shrinkability is moderate, and the heat A film having excellent shrinkability and excellent appearance after shrinkage was obtained, and a biaxially stretched film having excellent dimensional stability and good printability was obtained. This biaxially stretched film can be suitably used as a so-called low heat shrink overlapping packaging material, and can also be suitably used as food packaging such as ham and sausage, plastic bottles such as PET bottles, labels and caps for glass bottles, etc. It was a thing.
Comparative Example 1
The stretching temperature in the MD direction was 70 ° C., and the stretching ratio was 2.0 times. The stretching temperature in the subsequent TD direction was 78 ° C., and the stretching ratio was 3.5 times. Otherwise, a biaxially stretched film was obtained in the same manner as in Example 4.
[0038]
The physical properties and the like of the obtained film are shown in Table 1.
Comparative Example 2
The stretching temperature in the MD direction was 70 ° C., and the stretching ratio was 2.0 times. The stretching temperature in the subsequent TD direction was 76 ° C., and the stretching ratio was 3.0 times. Otherwise, a biaxially stretched film was obtained in the same manner as in Example 4.
[0039]
The physical properties and the like of the obtained film are shown in Table 1.
Comparative Example 3
The stretching temperature in the MD direction was 80 ° C., and the stretching ratio was 4.8 times. The stretching temperature in the subsequent TD direction was 85 ° C., and the stretching ratio was 3.0 times. Further, the relaxation rate in the lateral direction was set to 10%, and the heat setting treatment was performed at 120 ° C. for 10 seconds. Otherwise, a biaxially stretched film was obtained in the same manner as in Example 4.
[0040]
The physical properties and the like of the obtained film are shown in Table 1.
Comparative Example 4
As polylactic acid, the number average molecular weight is 90,000, MFR is 7.5 g / 10 min (210 ° C.), and the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 85. Polylactic acid (manufactured by Cargill Dow Polymer Co.) that was 0 / 15.0 (mol%) was used. Otherwise, an unstretched film was obtained in the same manner as in Example 1. The unstretched film obtained in the same manner as in Example 1 was subjected to simultaneous biaxial stretching treatment except that the preheating temperature in the stenter was 70 ° C. and the stretching temperature was 67 ° C. Next, an attempt was made to perform a heat setting treatment at 90 ° C. for 10 seconds with a relaxation rate of 5%, but the film could not be formed due to fusing during the heat setting treatment.
Comparative Example 5
In place of polylactic acid, aliphatic polyester (Showon Polymer Co., Ltd., Bionore 1903) was used. Otherwise, an unstretched film was obtained in the same manner as in Example 1. The unstretched film obtained in the same manner as in Example 1 was subjected to simultaneous biaxial stretching except that the preheating temperature in the stenter was 60 ° C. and the stretching temperature was 60 ° C. Next, an attempt was made to perform a heat setting treatment at 80 ° C. for 10 seconds with a lateral relaxation rate of 5%, but the film could not be formed due to fusing during heat setting.
[0041]
In Comparative Example 1, since the longitudinal draw ratio was low and the draw ratio was smaller than the range of the present invention, the heat shrinkability balanced in the biaxial direction was not obtained, the mechanical strength was low, and the printability was also low. It became inferior.
[0042]
In Comparative Example 2, since the longitudinal draw ratio was lower than the range of the present invention, the shrinkability was inferior, the appearance after packaging was not tight, and the printability was also inferior.
In Comparative Example 3, since the draw ratio was beyond the range of the present invention, the heat shrinkage ratio at a temperature lower by 10 ° C. than the melting point of the film was larger than the range of the present invention. Appearance deterioration such as bumpiness occurred, and the shrinkage was inferior.
[0043]
Since Comparative Example 4 used polylactic acid in which the blending ratio of L-lactic acid and D-lactic acid was outside the scope of the present invention, Comparative Example 5 used aliphatic polyester instead of the polylactic acid of the present invention. In either case, the stretched film was melted in the stenter and could not be formed into a film.
[0044]
【The invention's effect】
As described above, according to the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention, an appropriate biaxial heat can be obtained by performing biaxial stretching treatment under specific conditions without impairing mechanical properties. A film having shrinkability and excellent printability and shrinkability can be obtained. Such biaxially stretched film can be used for food packaging materials such as confectionery bags, packaging materials such as pharmaceuticals, low heat shrink overlapping packaging materials suitable for individual packaging or integrated packaging such as magnetic tapes and magnetic disks, plastic bottles, It can be suitably used as a label or cap used for glass bottles. In addition, when discarded as garbage, it is decomposed by microorganisms in the soil, and the environmental load on the natural environment and wild animals can be reduced.
[0045]
Moreover, according to the manufacturing method of the heat-shrinkable polylactic acid-based biaxially stretched film of the present invention, the polylactic acid-based biaxially stretched film of the present invention can be easily realized.

Claims

A biaxially stretched film made of a polylactic acid-based resin, wherein the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 100/0 to 94/6. The film has a heat shrinkage in the machine direction and transverse direction at 80 ° C. of 6% or less at 80 ° C., and at least in the machine direction or transverse direction of the film at a temperature 10 ° C. lower than the melting point of the film. The heat shrinkage rate in one direction is 30% or more, and the ratio of heat shrinkage rate between the vertical direction and the horizontal direction of the film at a temperature 10 ° C. lower than the melting point of the film is in the range of 1.0 to 1.3 . A heat-shrinkable polylactic acid-based biaxially stretched film.

The film has a maximum heat shrinkage stress of 0.7 MPa or more and a maximum heat shrinkage stress ratio of 1 to 1. The heat-shrinkable polylactic acid-based biaxially stretched film according to claim 1, which is in a range of 0.0 to 1.5 .

A polylactic acid resin mainly composed of polylactic acid in which the ratio of L-lactic acid to D-lactic acid is (L-lactic acid) / (D-lactic acid) = 100/0 to 94/6 (mol%) is made by melting. When the longitudinal stretch ratio of the film is X and the lateral stretch ratio is Y, the X is 2.5 times or more, the Y is 2.0 times or more, and the stretch ratio of X to Y The film is formed by biaxial stretching so that (X / Y) is in the range of 0.6 to 1.5 , followed by heat setting at 100 to 130 ° C. for 3 to 30 seconds, and further 2 to 10 % . A method for producing a polylactic acid-based biaxially stretched film, characterized in that a relaxation treatment is performed under the condition of% .